The oxidation of phenanthrene by rat liver microsomes significantly depends on the pretreatment of the animals as shown by means of gas chromatography/mass spectrometry. Whereas untreated animals convert phenanthrene exclusively into the 9,10-dihydrodiol (K-region), pretreatment with various polycyclic aromatic hydrocarbons and related compounds resulted in different rates of additional oxidation at the 1,2- and 3,4-position. Moreover, secondary metabolism to dihydrodiol epoxides, detected as triols, was observed. Despite considerable concentration of the proximate carcinogen of phenanthrene (1,2-dihydrodiol) only low concentrations of the ultimate carcinogen were detected which may explain the carcinogenic inefficiency of this hydrocarbon.